The present invention relates to the use of emulsion copolymers as fluid loss control agents in low solids drilling fluids and in dense drilling muds. The copolymers provide excellent fluid loss control to water-based drilling fluids and muds which contain large amounts of divalent cations such as calcium and magnesium.
In drilling subterranean wells and especially when employing rotary methods, drilling fluids or muds perform a variety of functions which influence such factors as the drilling rate, cost, efficiency and safety of the operation. Drilling fluids are circulated down through the drill shaft, out the bottom of the drill bit and up the annular space between the drill pipe and the formation wall to the surface where they may be mechanically or chemically treated and recirculated. The fluids provide lubrication at the point where the drill bit cuts into a geologic formation as well as along the length of the rotary drill pipe. As it lubricates the system, the fluid removes heat which is produced by the cutting action of the bit while additionally performing the important task of transporting cuttings produced by the system to the surface for removal. An acceptable drilling fluid must have body yet be free flowing with a measurable relatively low viscosity in order to be easily pumped. In addition, the fluid must have an acceptable gel strength in order to suspend the solid material if circulation is interrupted and to prevent accumulation of solids at the bit thus preventing mechanical jamming.
Drilling fluids must also provide sufficient equalizing pressure to contain naturally occurring liquid or gaseous components encountered in the formations pierced by the drill. By using proper mud formulations, possible collapses or blowouts resulting from uncontrolled influxes of formation fluids may be prevented.
Excessive loss of liquid to the formation results in increased fluid viscosities which causes decreases in drilling times in addition to poorer cuttings removal and lubrication. Therefore, in addition to preventing the flow of formation fluids into the hole, drilling fluids should also minimize the flow of liquid out into the formation. Both functions are served by the ability of the fluid to form a compressible, thin (but substantially impermeable) filter cake against exposed permeable formations composed of native solids found in the well bore and the solids present in the drilling fluid. In addition to being thin, a filter cake is also preferably smooth and firm yet flexible. Conversely, inferior filter cakes are thick, lumpy and/or brittle. If such deficient filter cakes are formed, additional problems such as abnormal pipe drag, differential pressure sticking, pressure surges, and swabbing will result.
The most common fluid loss control agents are clay solids present in the aqueous muds. By providing a wide particle size distribution of colloidal solids, a dense filter cake having both low porosity and low permeability is obtained as smaller size particles plug openings existing between the larger size partices. Bentonite clay, which contains a larger percentage of fine particles compared to other clays, is often employed as the primary filtration control agent.
When the use of clays alone does not lower the fluid loss of a drilling fluid to a sufficient level to eliminate damage to the formation as well as to the drilling equipment, organic colloids are often employed. The colloids hydrate in the aqueous fluid and proceed to bridge the gaps and pores between the clay particles resulting in a more impermeable filter cake. Typical organic colloids include, for example, pregelatinized starch, carboxymethyl cellulose and polyanionic cellulose. These materials are not without their disadvantages. For example, although possessing some salt tolerance, starches are known to be subject to bacterial attack and thermal degradation above 250.degree. F. Carboxymethyl cellulose is also subject to thermal degradation, and while known to be effective in calcium and sodium systems, it becomes less effective as salt concentrations exceed about 50,000 ppm. Polyanionic cellulose is used in salt systems due to its relatively high level of salt tolerance; however, this colloid often produces undesirable thickening or gelling of the drilling fluids which overshadows its salt tolerance.
Many water soluble homo- and copolymers based on vinyl monomers have also been found to be useful as fluid loss control additives:
In U.S. Pat. No. 2,650,905 (issued Sept. 1, 1953 to R. Fordyce et al.), use of sulfonated polystyrene polymers was shown to aid in forming substantially impermeable films around the walls of the bore hole while at the same time not affecting the viscosity of the muds.
In U.S. Pat. No. 4,455,240 (issued June 19, 1984 to C. A. Costello) drilling fluids treated with carboxylic functional polyampholytic co- or terpolymers containing anionic, cationic and optionally nonionic mer units had decreased fluid losses.
U.S. Pat. No. 2,718,497 (issued Sept. 20, 1955 to W. Oldham et al.) describes dry or solution copolymers containing carboxylic acid salt groups in a ratio of one carboxylic acid salt group to each two to six linear chain carbon atoms to provide fluid loss control to aqueous drilling muds. Applicable polymers included those obtained by polymerizing unsaturated aliphatic monocarboxylic acids such as acrylic or methacrylic acid. Hydrolyzed and saponified polymers and copolymers of acrylamide, acrylonitrile and the alkyl esters of acrylic acid were also described as useful. The additives were said to sequester only a limited quantity of calcium and other alkaline earth metal ions.
U.S. Pat. No. 2,775,557 (issued Dec. 25, 1956 to R. Morgan) describes the use of water-soluble salts of acrylamide-acrylic acid copolymers as wall sealing agents with improved water-retaining properties in a drilling mud while not increasing the viscosity of the mud. Copolymers having acrylamide:acrylate molar ratios of 60:40 to 75:25 were shown to provide fluid loss control in salt contaminated muds containing saturated sodium chloride or slightly soluble calcium sulfate or calcium hydroxide, for example.
Though not directed to fluid loss control, U.S. Pat. No. 3,323,603 (issued June 6, 1967 to J. Lummus et al.) also describes the use of acrylate-acrylamide copolymers in low solids drilling fluids. Small amounts (i.e., 0.005-0.15 pounds per 42-gallon barrel (ppb)) of the highly hydrolyzed polymers added to systems containing low concentrations of bentonite and less than 500 ppm Ca++ ion were found to beneficiate the bentonite while flocculating other clays present in the system.
In U.S. Pat. No. 3,072,569 (issued Jan. 8, 1963 to F. Siegele) fluid loss control agents which are useful in clay based drilling muds having calcium ion contents of at least 500 ppm at elevated temperatures are described. The agents are comprised of a substantially homogenous linear hydrocarbon chain containing both hydroxyl and carboxylic acid groups such as, for example, copolymers of vinyl acetate and either a carboxylic acid monomer or derivative, which upon hydrolysis yield hydroxyl and carboxylic acid salt groups, respectively.
Vinyl sulfonate-vinylamide polymers have also been used as fluid loss control agents. These polymers, containing anionic sulfo groups, have been described to be insensitive to calcium ions and as such have found usefulness in drilling through electrolyte-releasing formations.
Contamination by divalent cations, such as calcium and magnesium, occurs when drilling through electrolyte-releasing formations which contain gypsum, lime, and other salt deposits. If only small quantities of calcium ion are encountered, drilling muds are often treated with a carbonate such as soda ash to precipitate the ions in order to increase the efficiency of the fluid loss control agents employed. This is a costly time-consuming step which is not effective when larger quantities of more water-soluble calcium and magnesium salts (i.e. CaCl.sub.2 and MgCl.sub.2) are encountered, especially in the formation brines of such notable basins as Williston Basin, North Dakota and Anadarko Basin, Oklahoma. Oftentimes, fresh or sea water muds are actually calcium treated with quantities of gypsum or lime in order to alleviate problems that are associated with drilling in water-sensitive shale or clay-bearing formations.
There is therefore a need for fluid loss control agents which are compatible in salt-contaminated drilling fluids. If an additive is incompatible in a contaminated system, dramatic increases in fluid viscosity or clay flocculation may result. If gel formation results, the formation wall may easily become plugged thereby preventing oil from later flowing freely into the producing zone.
For the above reasons, those in the drilling field continue to search for agents that provide improved fluid loss control to drilling fluids and muds which contain high concentrations of divalent cations.
It is an objective of the present invention to provide drilling fluid additives which provide improved fluid loss control in drilling fluids containing large quantities of calcium as well as other divalent ions.